A unique probe designed to acquire nuclear magnetic resonance difference spectra of two samples is presented. The NMR Difference Probe contains two sample coils in a resonant circuit that switches between parallel excitation and serial acquisition to cancel common signals such as solvent peaks and impurities. Two samples containing a common analyte, acetonitrile, were used to demonstrate signal cancellation in a difference spectrum collected with a single pulse experiment. The cancellation was over 96% effective. The approach described has applications in the areas of solvent subtraction and spectral simplification.

Several experimental techniques have been developed to utilize spin-polarized xenon gas for sensitivity and selectivity enhancement in surface studies using solid-state NMR. Although previously reported as a viable spin polarization transfer mechanism, the details of high-field cross-polarization (CP) have not been thoroughly investigated. We recently reported observations of CP from an adsorbed layer of hyperpolarized xenon (HP Xe) to a variety of surface nuclei at temperatures as high as 323 K [J. Am. Chem. Soc. 105 (2001) 1412]. In this paper, we investigate many of the issues associated with HP Xe surface CP studies, including polarization transfer kinetics and the effects of temperature on the dynamics. Protonated and methylated silica samples are used as model systems for comparison. A comparison of the rate analysis data from CP and SPINOE (Spin Polarization-Induced Nuclear Overhauser Effect) experiments provides information on the origin of the difference in polarization transfer efficiencies between the two techniques. Lineshape analysis of 1 H spectra for CP and SPINOE experiments demonstrates the difference in selectivity of methods due to longer SPINOE evolution times that lead to greater spin diffusion. The results of this work help to assess the viability of HP Xe CP as a surface analysis technique.

Two improved approaches for the rapid analysis of multiple samples using multiplex sample NMR are described. In the first approach, frequency-selective 90 degrees radio frequency pulses and large pulsed field gradients are applied to excite and detect multiple samples in rapid succession. This method is advantageous for samples with relatively long longitudinal (T1) relaxation times. In the second approach, chemical shift imaging is applied to acquire both the spectral and spatial information of multiple samples simultaneously. Chemical shift imaging is more time-consuming than selective excitation; however, it is advantageous for detecting samples with short T1's and for signal averaging. Both approaches demonstrate the potential of multiplex sample NMR for carrying out high-throughput NMR detection.

ADVERTISEMENT RETURN TO ISSUEPREVCommunicationNEXTHyperpolarized Xenon-Mediated Cross-Polarization to Material Surfaces Observed at Room Temperature and AboveJay Smith, Luis J. Smith, Kevin Knagge, Ernesto MacNamara, and Daniel RafteryView Author Information H. C. Brown Laboratory, Department of Chemistry Purdue University, West Lafayette, Indiana 47907 Cite this: J. Am. Chem. Soc. 2001, 123, 12, 2927–2928Publication Date (Web):March 3, 2001Publication History Received5 October 2000Revised9 February 2001Published online3 March 2001Published inissue 1 March 2001https://pubs.acs.org/doi/10.1021/ja0056670https://doi.org/10.1021/ja0056670rapid-communicationACS PublicationsCopyright © 2001 American Chemical SocietyRequest reuse permissionsArticle Views176Altmetric-Citations8LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-AlertscloseSupporting Info (1)»Supporting Information Supporting Information SUBJECTS:Adsorption,Materials,Polarization,Silica,Zeolites Get e-Alerts

The cross-relaxation rate constants for a three spin system involving hyperpolarized Xe-129, C-13, and H-1 nuclei have been determined over a wide temperature range (136-216 K) through observation of the polarization enhancements from spin polarization induced nuclear Overhauser effect (SPINOE) experiments. A model system of methoxy groups adsorbed on high surface area silica was studied. A maximum polarization enhancement of 3.4 was observed for the methoxy carbon. Xenon-carbon and xenon-proton cross-relaxation were found to be dependent on both the xenon coverage and xenon diffusion across the silica surface. The temperature dependence of the cross-relaxation rate constants was effectively fit using a hopping model. Possible improvements for the SPINOE experiment are suggested including alternative substrates which would retard xenon diffusion across the surface and improve xenon-carbon cross-relaxation leading to significantly larger SPINOE enhancements.

We present an analysis of the cross-relaxation dynamics between optically polarized 129Xe and surface 13C and 1H spins in methanol adsorbed on a SiO2 surface. Analytical expressions for the 13C polarization are derived, which show that the 13C polarization is determined by cross-relaxation with both 129Xe and 1H spins. The 1H spins act as a polarization sink; however, saturation of the protons results in a further increase in signal intensity. Replacing the 1H spin reservoir with 2H increases the 13C signal by a factor of 12 (25 as compared to room-temperature data). Further improvements are anticipated using higher-powered lasers.

A new, pulsed nuclear quadrupole resonance (NQR) spectrometer capable of performing a variety of pulsed and swept experiments is described. The spectrometer features phase locked, superheterodyne detection using a commercial spectrum analyzer and a fully automatic, computer-controlled tuning and matching network. The tuning and matching network employs stepper motors which turn high power air gap capacitors in a “moving grid” optimization strategy to minimize the reflected power from a directional coupler. In the duplexer circuit, digitally controlled relays are used to switch different lengths of coax cable appropriate for the different radio frequencies. A home-built pulse programmer card controls the timing of radio frequency pulses sent to the probe, while data acquisition and control software is written in Microsoft Quick Basic. Spin-echo acquisition experiments are typically used to acquire the data, although a variety of pulse sequences can be employed. Scan times range from one to several hours depending upon the step resolution and the spectral range required for each experiment. Pure NQR spectra of NaNO2 and 3-aminopyridine are discussed.

A dual channel probe for the simultaneous acquisition of NMR data from multiple samples has been developed. This multiplex probe consists of two noninteracting sample coils that are each capable of detecting NMR signals at the same resonant frequency with good sensitivity and resolution.13C free induction decays for the two samples, methanol (13C, 99%) and carbon tetrachloride (13C, 99%), were acquired simultaneously at 75.44 MHz using a single transmitter pulse and separate NMR receivers.S/Nmeasurements are comparable to those observed using single coils. No evidence of cross talk is evident in the spectra even after considerable signal averaging. The probe demonstrates the feasibility of significant parallelism in NMR, which will be of interest in situations where high throughput analysis is desired.

The high polarization of optically pumped Xe-129 was transferred to surface carbon and silicon species under magic angle spinning (MAS) conditions to afford enhanced solid-state NMR of surfaces. High-resolution Si-29 MAS spectra of fumed silica and C-13 spectra of chemisorbcd methanol on silica were obtained using a steady flow of hyperpolarized xenon adsorbed onto the surface at 135 K. Cross polarization to Si-29 from SPINOE enhanced hydroxyl protons and to C-13 from enhanced methyl protons is observed with good efficiency. A direct SPINOE transfer from Xe-129 to C-13 without cross polarization is observed to give the highest enhancement under these conditions.

Improvements in the Multiplex Sample NMR method are investigated to explore its capabilities of analyzing multiple samples simultaneously. Issues of quantitation and resolution in the multiple-coil probe are examined in one- and two-dimensional experiments. Improvements in quantitation are shown to result from the use of reference deconvolution for one-dimensional experiments, while the use of two-dimensional methods has much improved resolution and shows the potential for significantly increased parallelism. A multiplicative scheme is shown to be an easily implemented, effective method for generating individual sub-spectra from individual samples.

A new parallel coil NMR probe is described which can detect simultaneously the signals arising from four individual samples. The signals are differentiated using field gradients and analyzed in software yielding either a table of peaks assigned to each sample or individual spectra. When suitably developed, this method should be useful for increasing the throughput of NMR measurements, an important issue in applications such as process monitoring or the screening of large molecular libraries using NMR detection.

Two-dimensional C-13 solid-state nuclear magnetic resonance (2D SSNMR) spectroscopy was applied to the study of three conformational polymorphs of 5-methyl-2- [(2-nitrophenyl)amino]-3-thiophenecarbonitrile. The separation of sidebands by order experiment (de Lacroix, S. F.; Titman, J. J.; Hagemeyer; A.; Spiess, H. W. J. Magn. Reson. 1992, 97, 435-443) allowed the separation of isotropic and anisotropic chemical shift information over two dimensions, making it possible to distinguish individual carbon atoms and analyze the full chemical shift anisotropy patterns using magic angle spinning (MAS) at moderate spinning speeds. The C-3 carbon of the thiophene ring, alpha to the nitrile carbon, was chosen to probe the differences in chemical environment between forms. The three forms exhibited a large variation in both measured and theoretically predicted isotropic and anisotropic chemical shifts for this carbon site. The results summarized in this paper demonstrate the utility in using 2D SSNMR methods for studying polymorphism in crystalline compounds with moderate molecular weights.

ADVERTISEMENT RETURN TO ISSUEPREVCommunicationNEXTOptical Pumping and Magic Angle Spinning: Sensitivity and Resolution Enhancement for Surface NMR Obtained with Laser-Polarized XenonDaniel Raftery, Ernesto MacNamara, Gregory Fisher, Charles V. Rice, and Jay SmithView Author Information H. C. Brown Laboratory, Department of Chemistry Purdue University, West Lafayette, Indiana 47907-1393 Cite this: J. Am. Chem. Soc. 1997, 119, 37, 8746–8747Publication Date (Web):September 17, 1997Publication History Received20 June 1997Revised7 August 1997Published online17 September 1997Published inissue 1 September 1997https://pubs.acs.org/doi/10.1021/ja972035dhttps://doi.org/10.1021/ja972035drapid-communicationACS PublicationsCopyright © 1997 American Chemical SocietyRequest reuse permissionsArticle Views442Altmetric-Citations71LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose SUBJECTS:Magnetic properties,Nuclear magnetic resonance spectroscopy,Polarization,Quantum mechanics,Silica Get e-Alerts